Silicon carbide semiconductors have excellent characteristics with a larger dielectric breakdown voltage, a wider energy bandgap and a higher thermal conductivity than silicon semiconductors. Therefore, silicon carbide semiconductors are expected to be applied to light-emitting devices, high-power devices, high-temperature-tolerant devices, radiation-proof devices, high-frequency devices and the like.
Incidentally, as devices (SiC semiconductor devices) in which the above-mentioned silicon carbide semiconductor is used, Schottky barrier diodes are known. The Schottky barrier diode is a diode in which rectification (Schottky effect) generated in the junction surface between a metal and a semiconductor is used, and has characteristics such that the forward drop voltage is lower than that of a PN diode, and the reverse recovery time is shorter than this. Therefore, the Schottky barrier diode is often used in a power supply circuit such as a switching regulator that performs high-frequency switching.
However, the Schottky barrier diode has a drawback in that the reverse leakage current is larger than that of the PN diode, and the reverse withstand voltage is lower than this. Therefore, in the Schottky barrier diode in which a SiC semiconductor device is used, there has been a problem in that an ohmic electrode which has a low contact resistance is formed in order to increase the forward current, and the reverse leakage current is reduced.
Generally, in order to obtain an ohmic electrode which has a low contact resistance, it is known that nickel (Ni) may be used as an electrode (see, for example, PTLs 1 or 2). In addition, it is known that after Ni is formed on the surface of a SiC substrate, heat treatment thereof at high temperature is effective in obtaining an ohmic electrode which has a low contact resistance.
On the other hand, various investigations have been made in order to reduce the reverse leakage current. For example, reduction in crystal defects of a micropipe or the like existing in a substrate or an epitaxial layer, a guard ring structure for preventing field effect concentration of the Schottky electrode end, and the like are proposed. Further, the impurity concentration of a SiC layer at the Schottky electrode side, or the height of the Schottky electrode, and the like have been investigated (see PTLs 3 to 5).